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Note: These instructions are for OS X or FreeBSD hosts. You'll need to interpret them differently on Linux, and some commands may be missing from your base system.

Update: Blackcat Studios rewrote the script to work with Linux. I've attached a copy at the end of the page.

Backtrack 5 R3 was recently released and I decided it was a good time to rebuild my USB key. I'd had occasions where I wanted to use it to boot different OSs instead of just Backtrack so I decided I'd go with the Multiboot ISO via GRUB2 method. Unfortunately I found that BT5 *still* doesn't support the iso-scan/filename parameter. No matter. We'll just respin the initrd.

Update: The fix is a little different and more complicated, for R3 so I replaced most of this guide with a shellscript (attached).

First download the shellscript and mount the iso somewhere.

$ chmod +x ./rebuild-initrd.sh

$ ./rebuild-initrd.sh /Volumes/BT5/casper/initrd.gz

Now you have a patched initrd.gz. Let's set up the USB key.

Partition it however you want, but take note: the core.img file that grub-install copies to the boot sector is 24172 bytes long - I had to start my first partition on sector 2 in order for it to install.

Format the key (i'm using vfat, ntfs or anything that grub2 supports should work), mount, install grub and copy your initrd into the boot directory that is created:

$ mkfs.vfat /dev/sdb1 -n KNOWLEDGE

$ mount /dev/sdb1 /media/KNOWLEDGE

$ grub-install --root-directory=/media/KNOWLEDGE /dev/sdb

$ cp ../initrd.gz /media/KNOWLEDGE/boot

And of course copy the bt4 iso to the key:

$ mkdir /media/KNOWLEDGE/iso

$ cp ~/Downloads/bt5-gnome-32.iso /media/KNOWLEDGE/iso

grub2 is configured by editing the /boot/grub/grub.cfg file. Here's my section for bt4: